Such reference systems are used, especially in neurosurgery, to localize a section of the body to be treated by means of determining the coordinates and to be able to accurately redetermine its position for treatment at any time.
An example for the use of such a reference system is a head ring, which is used for the stereotactic treatment of tumors, vascular deformities or the like present in the human brain. In this case, the head ring is fixed on the head of the patient, a brain tumor that is possibly present is localized by means of computerized tomography and its position with regard to the head ring is determined exactly, whereupon treatment instruments can be placed onto the head ring. Such treatment instruments can then also be available at the precise point for invasive or noninvasive therapy by means of a system of coordinates plotted against the reference system, i.e., the head ring in this case.
According to the state of the art, the reference systems--described here as a head ring by way of example--have vertical rods attached to the head ring, and tapholes are cut into the upper ends of the vertical rods. By means of these tapholes, fixation pins with a point are screwed in from the outside in the direction of the head, which penetrate slightly into the bony structures of the head while the head ring is being applied and fixed to the head through the scalp and thus can guarantee a firm fit of the head ring.
Various embodiments of such fixation pins are well known. Common prerequisites for the function of these fixation pins are:
In the first place, the pins must be stable enough to be able to provide a reference system with a secure hold. Furthermore, they should be made of a low-density nonmagnetic material in order to exert as few interferences as possible in a tomograph during the localization of the diseased section. To be able to produce the fixation pins at a low cost and to satisfy the above prerequisites at the same time, these are usually produced with a shaft made of aluminum or plastic. In this case, the materials used for the points include aluminum, titanium, or even steel. However, points made of soft materials, e.g., restrict the reusability of such fixation pins to the extent that these pins may only be used once and must be disposed of thereafter.
Another disadvantage of these prior-art fixation pins concerns the interfering effect of the metal parts used in them on the computerized tomography and nuclear spin tomography images to be produced. Especially because of metals, particularly if steel points are used, the images obtained appear to be distorted or of low quality on the tomography interference patterns, which stem from the fixation pins and frequently if the pins were arranged awkwardly during the first application of the head ring, such that the entire image can no longer be used, and the procedure must be repeated with a head ring attached to other points.
In addition, the use of fixation pins that can only be used once results in avoidable waste, while, on the other hand, the procurement costs are high, if new fixation pins must be used for each attachment of a head ring.